The indoor Paris Masters, usually an afterthought in the men's tennis calendar, features an exciting—and unexpected—lead story line this year: Roger Federer fighting for a berth in the exclusive and lucrative year-end tournament in London next week.

The ATP World Tour Finals invites the eight highest-ranked men healthy enough to play. Big purses and gobs of ranking points make it just as much a can't-miss event for the game's stars as are the U.S. Open and the other Grand Slam tournaments.

And Federer, even in decline at age 32, remains a huge draw—especially in London, where fans have watched him win seven Wimbledon titles.

Federer has qualified every year since 2002 and won six times. But this year, Federer's spot isn't quite guaranteed. He must win his opening match at the Palais Omnisports de Paris-Bercy on Wednesday, against Kevin Anderson, to make it to London. If Federer loses, his fate will hinge on rivals' results.

This raises the profile of Paris, which usually is overshadowed by the other big stops on the tennis tour, including the other eight Masters Series events, five of them key tuneups for Grand Slams. Two others—Indian Wells and Miami—are mini-Grand Slams, with oversize draws, two-week spans and women's tournaments. Shanghai is the premier event in Asia, a rapidly growing tennis market.

Bercy is the only indoor Masters event, and the only one that isn't the biggest tennis tournament in its city: Roland Garros, held five months earlier at the other end of Paris, easily trumps it. Last year Federer and Rafael Nadal skipped it. Novak Djokovic and Andy Murray combined to win just one match.

Federer could have punched his ticket to London on Sunday in his hometown of Basel, Switzerland. But instead he lost in the final to Juan Martin del Potro, 7-6 (3), 2-6, 6-4. After losing to del Potro in last year's Basel final, Federer withdrew from Bercy with his London berth secure. This year, he was practicing in Paris the next day.

Federer's talent can obscure his match tactics and strategic scheduling. His latest maneuver, playing Paris despite having all but clinched London, has already paid off before his first match.

His three biggest rivals for the last three spots in London are seeded seventh, eighth and ninth in Paris. Federer is ranked ahead of them, so if he had withdrawn they would have moved up to Nos. 6, 7 and 8. That would have made it possible for each to advance to the semifinals, since the 5, 6, 7 and 8 seeds are placed in separate quarters of the draw.

Instead, two Frenchmen, No. 8 Jo-Wilfried Tsonga and No. 9 Richard Gasquet, were slated to meet in the third round, meaning only one could earn enough points to pass Federer in the race to London. (Tsonga rendered the point moot by losing his opening match late Tuesday to Kei Nishikori.)

These arcane draw details matter because the tour finals matter. The tournament doesn't have the history of Wimbledon, or even its geographical fixity—Federer has won it in three different cities.

But undefeated champions earn 1,500 ranking points, three-quarters the haul of a Grand Slam winner. That bundle from his 2011 title helped Federer reclaim the No. 1 ranking last year. Djokovic, last year's winner, used the points to fend off Nadal's bid for the top spot until late this year.

The event's exclusivity also allows players just inside the top eight to widen their lead over those outside it.

In 2011, Tsonga didn't qualify for London until the week before. Then he reached the final and won 800 ranking points. That kept his ranking high enough, and his draws safe enough, to return to London last year despite having lost 12 of 13 matches against Top 10 players.

A decade ago, Federer completed his climb into the exclusive top ranks of the tour by winning the tour finals. Now, at the other end of his career, he and other veterans aren't loosening their grip on the sport's upper rungs. If the current standings hold, the eight contenders next week will average 28 years, four months in age—31/2 years older than the field at the 2003 tour finals in Houston. Five players that year were under 24, or more than a year younger than del Potro, likely the youngest London contender next week.

And if Federer qualifies and excels, his success will improve his chances of making it back to London this time next year, past his 33rd birthday.

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